X/gamma-ray emission from hot accretion flows in AGNs

Abstract

We present preliminary results of our study of the impact of strong gravity effects on properties of the high energy radiation produced in accretion flows around supermassive black holes. We refine a model of the X-ray emission from a hot optically-thin flow by combining a fully general-relativistic (GR) hydrodynamical description of the flow with a fully GR description of Comptonization. We find that emission from a flow around a rapidly rotating black hole is dominated by radiation produced within the innermost few gravitational radii, the region where effects of the Kerr metric are strong. The X-ray spectrum from such a flow depends on the inclination angle of the line of sight to the symmetry axis, with higher angles characterised by a harder slope and a higher cut-off energy. Under the (reasonable) assumption that the equatorial plane of a rotating supermassive black hole is aligned with the surrounding torus, these predicted properties may provide a crucial extension of the unified model of AGNs, allowing to reconcile the model with systematic trends reported in a number of studies of the X-ray spectral properties of AGNs (indicating that type 2 objects are harder than type 1 and that the relative amount of the reflected radiation is larger in the latter). On the other hand, the model with a rapidly rotating black hole predicts larger apparent luminosities for objects observed at higher viewing angles, while an opposite property (i.e. type 1 objects being more luminous than type 2) was revealed in the Integral data. Comment: 6 pages, proceedings of "The Extreme sky: Sampling the Universe above 10 keV"

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arXiv:1002.1915v1 [astro-ph.HE] 9 Feb 2010
X/γ-ray emission from hot accretion flows in AGNs
A. Nied´ zwiecki∗,aF.-G. Xiebcand A. A. Zdziarskid
aDepartment of Astrophysics, University of Łód´ z, Pomorska 149/153, Łód´ z, Poland
bKavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
cKey Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory,
Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, China
dCopernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland
E-mail: niedzwiecki@uni.lodz.pl,fgxie@shao.ac.cn, aaz@camk.edu.pl
We present preliminary results of our study of the impact of strong gravity effects on proper-
ties of the high energy radiation produced in accretion flows around supermassive black holes.
We refine a model of the X-ray emission from a hot optically-thin flow by combining a fully
general-relativistic (GR) hydrodynamical description of the flow with a fully GR description of
Comptonization. We find that emission from a flow around a rapidly rotating black hole is domi-
nated by radiation producedwithin the innermost few gravitational radii, the region where effects
of the Kerrmetric are strong. The X-rayspectrum fromsuch a flow dependson the inclination an-
gle of the line of sight to the symmetry axis, θobs, with higher θobscharacterised by a harder slope
and a higher cut-off energy. For a non-rotating black hole, dependence on θobsis insignificant.
Under the (reasonable) assumption that the equatorial plane of a rotating supermassive black hole
is aligned with the surrounding torus, these predicted properties may provide a crucial extension
of the unified model of AGNs, allowing to reconcile the model with systematic trends reported in
a number of studies of the X-ray spectral properties of AGNs (indicating that type 2 objects are
harder than type 1 and that the relative amount of the reflected radiation is larger in the latter). On
the other hand, the model with a rapidly rotating black hole predicts larger apparent luminosities
for objects observed at higher θobs, while an opposite property (i.e. type 1 objects being more
luminous than type 2) was revealed in the Integral data.
We investigate also the hadronic γ-ray emission from hot flows and we find much higher (by
orders of magnitude) γ-ray luminosities than estimated in previous studies. If nearby AGNs
containrapidlyrotatingblack holes andweakly magnetizedhot flows, their γ-rayemission should
be detectable by current γ-ray detectors.
The Extreme sky: Sampling the Universe above 10 keV
October 13-17, 2009
Otranto (Lecce) Italy
∗Speaker.
c ? Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlikeLicence.
http://pos.sissa.it/

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X-ray and γ-ray emission from AGNs
A. Nied´ zwiecki
1. Introduction
A large amount of the X-ray data, gathered over the last two decades, indicate that various
kinds of the geometry of accretion flows occur in AGNs, with an optically thick disc extending
down to the black hole horizon, truncated at several tens of gravitational radii or completely absent
in the central region. We focus here on the last case, characteristic, in particular, of radiogalax-
ies. For these objects, the most likely solution of accretion flow is that of an optically thin, two-
temperature flow, extensively studied in the context of radiatively inefficient objects. Models of the
X-ray emission from such flows typically use certain approximations, in particular, any relativistic
effects are neglected, in spite of the (most likely) origin of this emission in the strongly relativistic
region close to the black hole where most of the gravitational energy is dissipated. Such simplified
models do not allow to study some relevant properties of the X-ray spectra, discussed below.
Proton-proton collisions in two-temperature flows lead to substantial γ-ray emission through
neutral pion production and decay. Recently, observations by AGILE and Fermi significantly ad-
vanced our exploration of the γ-ray activity of AGNs. Motivated by this, we consider the hadronic
γ-ray emission in the previously unexplored regime of a weakly magnetized flow around a rapidly
rotating black hole (the former property is supported by the results of numerical simulations of
accretion driven by magnetorotational instability and the latter by some evolutionary scenarios of
supermassive black holes).
2. The models
We consider a supermassive black hole, characterised by its mass, M, and angular momentum,
J, surrounded by a geometrically thick accretion flow with an accretion rate ˙ M. The following
dimensionless parameters are used in this paper: r = R/Rg, a = J/(cRgM), ˙ m = ˙ M/˙ Medd, where
Rg= GM/c2,˙ Medd= Ledd/c2and Ledd≡ 4πGMmpc/σT.
We solve equations of the dynamical structure of the flow and then we use the computed
profiles of density, temperature and velocity field in modeling the leptonic and hadronic radiative
processes. Our model is similar to our previous study [1], with three major differences: (1) our
dynamical description of the flow is fully GR here, while in [1] we use non-relativistic hydrody-
namical equations; (2) the procedure (developed in [1]) for finding the self-consistent structure of
the flow with non-local Compton cooling rate has not been implemented in the model described
here yet; (3) we neglect here direct viscous heating of electrons. Weassume the viscosity parameter
α = 0.3 and the magnetic pressure equal 1/10th of the total pressure.
The Comptonization process is modeled using a Monte Carlo method, see [2]. The seed
photons are generated from synchrotron and bremsstrahlung emissivities of the flow; their transfer
and energy gains in consecutive scatterings are affected by both the special relativistic (SR) and
gravitational effects. In computing the hadronic component, we assume that in the plasma rest
frame the spectrum of γ-ray photons corresponds to the emission of thermal protons population.
We model such a thermal emission strictly following [3] and then we compute the transfer of γ-ray
photons in curved space-time. Absorption of γ-rays to pair production is negligible for parameters
considered in this paper.
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X-ray and γ-ray emission from AGNs
A. Nied´ zwiecki
Figure 1: The radial profiles of the proton (a) and electron (b) temperature, the half-height (c), the vertical
optical depth (d) and the azimuthal (e) and radial (f) velocities of our hot flow solutions. In all panels, the
black, blue and red curves are for model a1, a1,oand a0, respectively.
We consider two models without an outflow, i.e. with constant ˙ m(r)(= ˙ m0), with extreme
values of the spin parameter, a = 0 and a = 0.998, denoted as model a0and model a1, respectively.
For a = 0.998 we consider also a model with an outflow, with radius-dependent accretion rate,
˙ m(r) = ˙ m0(r/200)0.3, denoted as model a1,o. In all models ˙ m0= 0.1. We assume M = 2×108M⊙,
except for Fig. 2c where models with M = 2×108M⊙and 5×106M⊙are compared.
3. Results
Rotation of the black hole affects the structure of the flow in a manner established in previous
studies, see Fig. 1. Geometrically thick flows are in general sub-Keplerian, however, for a = 0 a
rapid radial acceleration occurs around r = 10, while the black hole with a = 0.998 stabilizes the
circular motion of the innermost part of the flow and the radial velocity remains rather small down
to r ≃ 2. Then, the continuity equation implies a much higher value of the optical thickness of the
innermost flow around a rapidly rotating black hole (note that model a0yields similar τ as model
a1,oalthough in the latter most of the matter is lost to the outflow). This, in turn, yields a higher
electron temperature (required to cool electrons) for a = 0. Finally, a more efficient dissipation of
gravitational energy occurs in models with a = 0.998, resulting in much higher proton temperature
at r < 10 in these models.
These differences in the structure of the central region result in a strong difference of the
properties of escaping radiation. For a = 0.998 the Comptonized component is much harder (by
∆Γ ≃ 0.2, Γ is the photon spectral index) and the radiative efficiency (due to electrons only), η, is
an order of magnitude larger than for a = 0. Specifically, η = 0.2% and 0.02% for model a1and
a0, respectively; for model a1,o, η is 4 times smaller than in model a1, which corresponds to the
fraction of the matter lost in the outflow.
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X-ray and γ-ray emission from AGNs
A. Nied´ zwiecki
Figure 2: Left panels: spectra of leptonic (between 10−3eV and several MeV) and hadronic (between 10
MeV and several GeV) emission from a hot accretion flow. In all panels, the black and the blue curves show
spectra averaged over all viewing angles with cosθobs∈ [0.7,1] and [0.3,0.6], respectively. The assumed
distance is D = 10 Mpc. Right panels: angular distribution of the flux between 2 and 10 keV (the black
curves) and the total flux in the γ-ray (hadronic) component (the red curves), shown as the ratio of the flux
observed from a given direction to the observed flux averaged over all viewing angles. Panels (a,d): model
a0; panels (b,e): model a1,o; panels (c,f): model a1. Panel (c) shows spectra for both M = 2×108M⊙(blue
and black) and M = 5×106M⊙(red and magenta, multiplied by a factor of 40).
Figs. 2(a-c) show the observed spectra from a hot flow, averaged over the ranges of interme-
diate and small θobs. Figs. 2(d-f) give more detailed information on the angular distribution of the
observed X-ray and γ-ray fluxes. The dependence on θobsresults from the combination of the SR
collimation and gravitational focusing of radiation. Both effects reduce the flux received by face-on
observers, more efficiently in high-a models. For a = 0, vr> vφand collimation toward the black
hole dominates over collimation toward the edge-on observers. For high a, an effect unique for the
Kerr metric, i.e. bending of photon trajectories to the equatorial plane, strongly enhances the flux
emitted to edge-on directions.
Theanisotropy ismorepronounced forthe hadronic component, production ofwhich isstrongly
centrally concentrated and which is thus subject to stronger relativistic effects. For the Comp-
tonized radiation, the anisotropy is slightly reduced due to mixing of radiation produced in the
innermost part with that from more distant regions.
The Comptonized component in model a1is dominated by radiation produced at r ≤ 2. Ra-
diation from that region has a strong intrinsic anisotropy (see [2]) and its properties are partially
reflected in the shape of the total spectrum, being slightly harder and having a higher cut-off en-
ergy at larger θobs. The effect is rather moderate, with the slopes of the edge-on and the face-on
spectra differing by ∆Γ = 0.1 and their cut-off energies differing by a factor of 2. For the average
face-on and intermediate spectra (shown in Fig. 2c; these are appropriate for comparison with the
stacked spectra of type 1 and 2 objects presented in studies discussed in Section 4) the difference is
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X-ray and γ-ray emission from AGNs
A. Nied´ zwiecki
even smaller, e.g. ∆Γ ≈ 0.05. On the other hand, global cooling effects should reduce the electron
temperature at larger r (see [1]), leading to weaker contribution of the isotropised radiation at high
energies; then, a more pronounced dependence of Γ on θobsmay be expected when these effects
are taken into account.
For a = 0.998, the γ-ray component contains a substantial part of the bolometric luminosity
of the flow. E.g., in model a1the total luminosity of the γ-ray component is only by a factor of 3
lower than the luminosity of Comptonized synchrotron and bremsstrahlung emission. The hadronic
production of γ-rays in hot flows was studied in [4] and [5] and much smaller γ-ray luminosities
were found. This was caused by much smaller proton temperatures in their models, which resulted
from the neglect of the black hole rotation in [4] and the assumed equipartition between magnetic
and gas pressures in [5]. Actually, scaling the results of [4] and [5] to the value of parameters of our
models, gives the levels of γ-ray luminosity several times higher than those in Fig. 2, most likely
due to the neglect of any relativistic effects affecting the γ-ray component in both [4] and [5].
4. Discussion
Studying effects of strong gravity near black holes is one of the key goals of high-energy
astronomy. So far, direct investigation of such effects focused on distortions of discrete spectral
features emitted from optically thick discs. We point out that relativistic models of radiative pro-
cesses in optically-thin flows predict some observationally testable effects, directly related to the
properties of the space-time of a rapidly rotating black hole. In our discussion of points (i) and (ii)
below, we assume that the midplane of a torus embedding the central region lies in the equatorial
plane of a central black hole, which is the most natural configuration for a rapidly rotating black
hole as the spin of the hole is aligned with the angular momentum of the outer disc on rather short
time scale, e.g. [6].
(i) Intrinsic anisotropy. The predicted anisotropy of the X-ray emission may have crucial
implications for the unification scheme of AGNs, where various classes of AGNs are supposed to
have the same central engine and the observed differences are attributed to orientation effects. In
the original formulation of this scenario, type 1 and type 2 objects produce the X-ray radiation with
the same intrinsic spectrum, but the latter are seen at larger inclination angles and their emission
is partially absorbed by a torus surrounding the central region. Then, a number of recent reports
that type 2 objects appear to be, on average, intrinsically flatter than type 1, seemed to contradict
the unification model. We point out that such a (reported) property should indeed characterize
the X-ray radiation from AGNs, if most of them contain rapidly rotating black holes. According
to our preliminary results (for a specific accretion scenario, specific parameters and neglecting
global cooling effects), the GR Comptonization model cannot explain the large difference of ∆Γ ≈
0.4 between of the average spectral slopes of Seyfert 1 and Seyfert 2 galaxies, derived from the
CGRO/OSSE ([7]) and Swift ([8]) data. Our predicted ∆Γ ≈ 0.05 approximately agrees with the
average slopes of type 1 and 2 objects found in the Integral (Γ =1.96 and 1.91; [9]) and BeppoSAX
(Γ = 1.89 and 1.80; [10]) data with more complex models, involving the reflection component;
however, the differences of these average spectral slopes are only marginally significant, which
may indicate that such a magnitude of ∆Γ is too small to be reliably verified with current detectors.
Both [9] and [10] find the average cut-off energies higher in type 2 objects than in type 1, consistent
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